What is a Sun Number score and how is it calculated?



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If you’ve ever searched for homes on Zillow, you may have come across something called a Sun Number. Sun Number was developed with the help of the U.S. Department of Energy's SunShot Grant to make understanding solar a little bit easier.

The Sun Number cuts out the confusing solar jargon and complex nuances that so often turn consumers off from going solar and instead provides a score ranging from 1 to 100 that represents how suitable a home is for going solar. 

Sun Number looks at four individual scoring categories, including how suitable a building is for solar, the regional climate, local electricity rates, and solar costs in the area. The higher the Sun Number, the better the conditions for installing solar panels.

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    Sun Number score components

    There are four individual scoring categories that make up the total Sun Number score: 

    • Building Solar
    • Regional Climate
    • Electricity Rate
    • Solar Cost

    Each of the categories is weighted differently based on how much it impacts a home’s suitability for solar. 

    1. Building Solar score

    Maximum score: 80 points

    The Building Solar score accounts for a majority of the total Sun Number and measures how suitable the actual structure of a home is for going solar. 

    The Building Solar score encompasses things like:

    Ideally, solar systems installed in the U.S. should be installed on south-facing roofs with decent square footage at an angle that matches the latitude of the home’s location, with zero shading. 

    The fact of the matter is that almost no homes will meet every single one of these criteria. But that doesn’t mean that solar won’t work - the system might just produce less electricity or require a more complex design. 

    Sun Number uses 3D imagery to determine how a home stacks up against these ‘perfect conditions’ and spits out the Building Score. You want the building score to be no less than 50, but it is best to have it higher than 60. 

    2. Regional Climate score

    Maximum score: 8 points

    The amount of sunlight that hits a home’s roof directly affects how much electricity solar panels can produce. Sun Number takes this into account through their Regional Climate Score. To calculate the Regional Climate score for a house, Sun Number uses solar irradiance data from the National Renewable Energy Laboratory (NREL). 

    The more sunlight an area receives, the higher its Regional Climate score will be.

    3. Electricity Rates score

    Maximum score: 8 points

    Next, Sun Number takes a look at average electricity rates in the area of the home. Solar panels can save homeowners more money in areas where the cost of electricity is high.

    If the area has high average electric prices, the Sun Number score will go up. Low electricity prices produce a lower Sun Number score. 

    Find out if going solar is worth it where you live

    4. Solar Cost score

    Maximum score: 4 points

    Sun Number also takes local solar installation prices into consideration; however, cost makes up the smallest portion of the total score. Sun Number finds local solar costs through NREL, and the lower local solar prices are, the higher the score. 

    However, you want to be wary of solar prices that are too low, as it may not end up being the highest quality installation. Our guide to finding the right price for solar can help you find a balance between low price and high quality

    Total Sun Number score

    Maximum score: 100 points

    The final Sun Number score is the sum of the Building Solar, Regional Climate, Electricity Rates, and Solar Cost scores. Although 100 is the ideal score, any Sun Number above 70 means solar is likely a worthwhile investment for that property.

    The final score will look something like this: 

    sample sun number score

    The sum of the four scoring categories gives you the total Sun Number score. Image source: Sun Number

    It will also show you the average Sun Number score for homes in your area.

    Does a low Sun Number score mean you can’t go solar?

    No, even if your Sun Number score is low, you can still make the switch to solar. The Sun Number score is designed to give a simple snapshot of a property’s solar potential, so it doesn’t take into account all of the small details of installing solar. 

    For instance, your home may have a Sun Number score of 60, but a solar installer may be able to design a system that can still offset most of your electricity usage by installing panels on multiple sides of your roof. 

    It may also be worthwhile to go solar if you live in an area with unreliable utility service. You can install enough solar to charge a solar battery that can power your home in the event of a power outage. While this might not be the most economical option, it may be worth it for the piece of mind reliable battery backup provides. 

    The bottom line is the Sun Number score shouldn’t be the end-all, be-all determining factor of whether or not you should go solar. It just serves as an easy way to get the process of installing solar started. 

    To really find out if solar is worthwhile for you, you need to consider a few more details, like the solar payback period and what solar incentives are available in your area. By using our solar panel cost and savings calculator, you can get more detailed insights into a solar installation for your specific home. 

    Calculate your solar panel payback period 
     - Author of Solar Reviews

    Catherine Lane

    Written Content Manager

    Catherine is the Written Content Manager at SolarReviews. She has been researching and writing about the residential solar industry for four years. Her work has appeared in Solar Today Magazine and Solar Builder Magazine, and has been cited by publications like Forbes and Bloomberg.

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